The present invention relates to machines for the production of filter rods which can be subdivided into filter mouthpieces for cigarettes, cigars or cigarillos. More particularly, the invention relates to improvements in devices for stretching tows of randomly crimped filamentary filter material in filter rod making machines.
The tow of filter material which is converted into the rod-like filler of a continuous filter rod consists of a very large number of crimped filaments, e.g., fibers made of acetate or another suitable synthetic plastic material. The tow is normally flat and is stored in the form of a highly compacted bale wherein the filaments in successive increments of the tow are crimped to a different extent. This creates problems in connection with the application of plasticizer which is used to bond neighboring filaments to each other in order to enable the filler of the filter rod to form a maze of minute passages for tobacco smoke. As a rule, the tow is withdrawn from the bale in an upward direction and is thereupon loosened (banded), stretched, sprinkled with atomized plasticizer, condensed into a rod-like filler, wrapped into a web of cigarette paper or the like, and subdivided into filter rod sections of desired length.
As mentioned above, the crimp of filaments which form the tow is not uniform. In many instances, the crimp varies within a very wide range so that the quantity of filter material in successive unit lengths of the tow also varies within such wide range. The differences in crimp of successive increments of the tow can be readily observed with the naked eye by looking at the layers of the tow in a bale, i.e., a strongly crimped portion of the tow is preceded or followed by a portion which is substantially free of crimps or by a portion whose crimp is much less pronounced than that of the preceding or next-following portion. On the other hand, it is desirable to insure that each and every unit length of the filler in a filter rod should contain identical quantities of filamentary filter material. The absence of homogeneousness in successive increments of the filler which forms part of a filter rod would cause the filter mouthpieces for cigarettes, cigars or cigarillos to offer a widely different resistance to the flow of tobacco smoke therethrough. Therefore, the manufacturers of filter rods strive to uniformize the tow before the latter enters the rod forming unit.
Presently known filter rod making machines already comprise devices which serve to stretch the tow upstream of the plasticizing station. For example, U.S. Pat. No. 3,224,453 to Mahoney et al. discloses a filter rod making machine wherein the tow passes through several pairs of rolls at least one pair of which is driven to draw the tow from the bale. The remaining rolls bear against the tow and are rotated as a result of frictional engagement with the filaments of the tow. It is also known to drive the first pair of rolls at a relatively low speed and to drive the next-following pair or pairs of rolls at a higher speed so that the filaments are stretched during travel from the nip of a preceding pair of rolls toward the nip of the next-following pair of rolls. It has been found that such arrangement does not suffice to insure satisfactory homogenization of the tow so that each and every increment of the rod-like filler in the filter rod will contain identical quantities of filamentary filter material.